Shichimotsu-koka-to (SKT) is a Kampo (traditional Japanese herbal) medicine, which is used in Japan to treat hypertension and atherosclerosis. We investigated the inhibitory effect of SKT on experimental pulmonary metastasis of B16 melanoma cells. The intake of SKT at a dose of 430 mg/kg for 6 weeks from 2 weeks before tumor inoculation significantly reduced the number of metastatic surface nodules in the lung and extended the life span. When the duration of SKT intake was examined, survival time was not affected by preintake before B16 melanoma cell inoculation and was slightly extended by postintake after B16 melanoma cell inoculation, although the life span was prolonged by intake throughout the experiment. To address the mechanism underlying the antimetastatic effect of SKT, we studied whether SKT modulated macrophage function, which is involved in killing tumor cells. The intake of SKT for 6 weeks dose dependently increased nitric oxide (NO) production by macrophages following stimulation with lipopolysaccharide. The elevated NO was found to serve as a cytotoxic mediator against B16 melanoma cells in co-culture with macrophages. On the contrary, B16 melanoma-conditioned medium reduced NO production by macrophages. However, SKT treatment reversed the reduction in NO production by the conditioned medium significantly. These findings may suggest that macrophage function-modulating activity by SKT appears to underlie its antimetastatic activity, which leads to a decrease in the number of lung metastatic surface nodules and the extension of life span.